Of oligomerization catalysts for .alpha.-olefins, many of the dimerization catalysts for .alpha.-olefins include a transition metal as an activity center thereof. The known dimerization catalysts include a catalyst system comprising a transition metal compound and an organoaluminum compound, a transition metal compound catalyst and a transition metal heterogeneous catalyst. Of these dimerization catalysts, the catalyst system comprising a transition metal compound and an organoaluminum compound is excellent in both catalytic activity an dimer yield.
A catalyst comprising a nickel compound and an organoaluminum compound is known as the dimerization catalyst comprising a transition metal compound and an organoaluminum compound. For example, Japanese Patent Publn. No. 19408/1987 discloses a process for dimerizing lower .alpha.-olefins, wherein there is used a catalyst comprising (A) a nickel compound, (B) a bis(dialkylaluminoxane) represented by a formula (R.sub.2 Al).sub.2 O (wherein R is alkyl of 1 to 6 carbon atoms or cycloalkyl), (C) a phosphorus-containing compound, and (D) a halogenated phenolic compound. However, catalysts capable of selectively forming both an .alpha.-olefin dimer and an .alpha.-olefin trimer were not known.
The present inventors have conducted intensive research in order to obtain a catalyst having high activity compared with conventional dimerization catalysts for .alpha.-olefins, and capable of producing specific dimers in a high yield and in a high selection ratio. As a result, the present inventors have found that a catalyst comprising at least one nickel compound selected from the group consisting of nickel salts of organic acids, nickel salts of inorganic acids and complex compounds of nickel, an organoaluminum oxy-compound having at least 3 aluminum atoms in the molecule and a trivalent phosphorous compound is excellent in catalyst activity, a dimerization yield and selectivity of the products. The present inventors have still further found that when ethene is used as .alpha.-olefin, such a catalyst as described above is capable of producing a trimer of ethene. The present invention has been achieved based on the finding described above.